In the cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of a metallic conductor cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures.
Optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not necessarily detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments and should be prevented.
Consequently, it should be no surprise that cables for transmitting communications signals must meet industry standards with respect to waterblocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield which often times is used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longitudinal seam. However, the forming of a shield about a cable core requires the use of relatively low manufacturing line speeds. Also, the use of a metallic shield is destructive of the otherwise all-dielectric property of an optical fiber cable. Further, lightning strikes may cause holes in a metallic shield.
It is not uncommon to include provisions in addition to or as an alternative to a metallic shield for preventing the ingress of water into the core. Waterblocking materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of such a material, which typically is referred to as a filling material and which typically is in the form of a grease-like composition of matter, causes housekeeping problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core. In optical fiber cables, a further important function of a filling material is the maintenance of the optical fibers in a low stress state.
A grease-like composition of matter typically is a semisolid or semiliquid substance comprising a thickening or gelling agent in a liquid carrier. The gelling agents used in greases frequently are fatty acid soaps, but high melting point materials, such as clays, silica, organic dyes, aromatic amides, and urea derivatives also are used. Nonsoap thickeners are typically present as relatively isometric colloidal particles. All types of gelling agents form a network structure in which the carrier is held by capillary forces.
When a low stress is applied to a grease-like material, the material acts substantially as a solid. If the stress is above a critical value, then the material flows and the viscosity decreases rapidly. The decrease in viscosity is largely reversible because it is typically caused by the rupture of network junctions between the filler particles, and these junctions can reform following the release of the critical stress.
A cable filling material, especially an optical fiber cable filling material, should meet a variety of requirements. Among them is the requirement that the physical properties of the cable remain within acceptable limits over a rather wide temperature range e.g., from about -40.degree. to about 76.degree. C. It is desirable that the composition of matter of the filling material be substantially free of syneresis, i.e. have an ability to retain uniform consistency, over the temperature range. Generally, syneresis is controlled by assuring dispersion of an adequate amount of collodial particles or other gelling agent. Other desirable properties of grease-like compositions include thermal oxidation resistance.
Further complicating the optical fiber cable situation is the introduction of a waterblocking filling material into the cable core in order to prevent the incursion of water. Suitable waterblocking materials in use must yield under strains experienced when the cable is made or handled. Otherwise, movement of the optical fibers within the cable would be prevented and the fibers would buckle because they contact, with a relative small periodicity, a surface of the unyielding filling material. The smaller the periodicity of the fibers when contacting such an unyielding surface, the greater a loss which is referred to as microbending loss.
Typically, microbending loss in optical fiber cables is more difficult to control at long wavelengths than at short ones. Thus the requirements on the mechanical properties of a fiber cable filling material are typically substantially more severe for cable that is to be used at 1.55 .mu.m, for example, than they are if the cable is to be used at shorter operating wavelengths of 1.3 .mu.m, for example. Although, it has been found that some prior art filling materials perform quite satisfactorily at wavelengths up to about 1.3 .mu.m, it has also been found that this is often not the case at longer wavelengths.
Because silica-based optical fibers typically have their lowest losses at or near the 1.55 .mu.m wavelength, there is great interest in operating optical fiber telecommunication systems at approximately that wavelength. Thus, it is important to have available optical fiber cable that has no significant cabling-induced losses at long wavelengths, including about 1.55 .mu.m.
Filling compositions for use in optical fiber cables should have a relatively low shear modulus, Ge. However, it has been determined that, at least for some applications, a low value of G.sub.e of the filling material is not sufficient to assure low cabling loss, and that a further parameter, the critical yield stress, .sigma..sub.c, needs to be controlled because it also affects the optical performance of fibers in a cable filled with a grease-like composition of matter.
A grease-like filling composition of matter having a relatively low critical yield stress is disclosed in U.S. Pat. No. 4,701,016 which issued on Oct. 20, 1987 in the names of C. H. Gartside, III, et al. and which is incorporated by reference hereinto. The composition comprises oil, a gelling agent such as colloidal particles, and, optionally, a bleed inhibitor. It includes 93% by weight mineral oil and 7% by weight of hydrophobic formed silica. Among oils useful in the practice of the invention are ASTM type (ASTM D-226 test) 103, 104A, or 104B (or mixtures thereof) naphthenic oils having a minimum specific gravity of about 0.860 and a maximum pour point (ASTM D97) of less than approximately -4.degree. C., and polybutene oils of minimum specific gravity of about 0.83 and a maximum pour point (ASTM D97) of less than about 18.degree. C. The colloidal particle filler material preferably comprises silica particles. Preferred bleed inhibitors are styrene-rubber or styrene-rubber-styrene block copolymers, and/or semiliquid rubbers, such as a high viscosity polyisobutylene. Other ingredients, such as, for example, a thermal oxidative stabilizer, may be present. The critical yield stress of the filling material of U.S. Pat. No. 4,701,016 is not greater than about 70 Pa, measured at 20.degree. C. whereas the shear modulus is less than about 13 kPa at 20.degree. C.
Incorporating a block copolymer into the grease-like composition of matter allows a reduction of the amount of colloidal particles that has been to added to the mixture to prevent syneresis of the gel. This reduction can result in cost savings. Furthermore, it makes possible the formulation of less bleeding compositions having a very low critical yield stress.
Waterproofing filling materials for use in cables also must pass industry standard drip tests. To pass these tests, filling materials in cable cores must be retained as cable samples, suspended vertically, are subjected to specified elevated temperatures. Some prior art materials, which have been used, perform satisfactorily with respect to microbending and associated losses, but they bleed out excessively and have problems in meeting current drip tests. Also, it is desired that the low means added losses exhibited by some prior art filling materials at least be met by filling materials which pass the drip test and have suitable low temperature properties.
Oil separation is a property of a grease-like material which describes the tendency to bleed oil during its lifetime. What is desired is a filling material which has an oil separation no greater than 30% when centrifuged at 10,000 rpm for one hour.
Because cable drip is related to oil separation, constraints on the sought after filling material include oil separation, critical yield stress and viscosity. The viscosity of the sought after filling material also is important with respect to processing. These constraints usually are antagonistic to each other. For example, a reduction of oil separation and an increase in cable drip temperature require high viscosity and yield stress whereas to facilitate processing and to reduce optical loss requires low viscosity and yield stress.
Another problem relating to filled optical fiber cables is the compatibility of the filling material with some coating materials which are disposed about drawn optical fiber to protect the optical fiber. If compatibility is lacking, the performance and/or the appearance of the optical fiber could be affected adversely. The compatibility of otherwise suitable prior art filling materials with some coating materials, particularly those which are relatively soft, is something less than desired.
Although some prior art compositions of matter are suitable for filling cable cores comprising optical fibers each having layers of particular coating materials thereon, the prior art does not appear to include a cable filling material which is suitable for filling cable cores which include optical fiber coated with some of the softer coating materials used today. What is sought after and what does not appear to be disclosed in the prior art is an optical fiber cable filling composition of matter which is compatible with a broad range of optical fiber coating materials, which does not bleed and which does not drip from the cable core at specified elevated temperatures and one which does not exacerbate optical loss.